1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, printer, etc., which forms images according to the electrophotographic system and an examination image forming method in such image forming apparatus.
2. Description of the Related Art
Electro-photographic copying machines, printers, etc. are widely known as image forming apparatus. For instance, in case of electro-photographic copying machines, etc., after uniformly charged by a main charger, the surface of a photo-conductive drum is exposed and an electrostatic latent image corresponding to a desired image is formed on the surface of the photo-conductive drum. Then, this electrostatic latent image is developed by a developer supplied from a developing device and a developer image is formed and further, this formed developer image is transferred on a transfer paper by a transfer charger and an image is formed thereon.
On this type of copying machine, in order for the stabilized image forming, the quality of a formed image is examined periodically and if it is out of a specified proper range, charging amount, exposing amount, etc. are adjusted or replacement of a photo-conductive drum is urged so as to prevent the quality change.
A method to judge the image quality by measuring the density of an examination image by an image density sensor after forming this examination image in a specified pattern on the surface of a photo-conductive drum and comparing it with a specified value has been known as a method to examine the image quality.
The formation of such an examination image consumes a developer irrelevant of a developer that is used for the normal image formation by user and it is therefore desirable to form an examination image as could as small and minimize the consumption of developer.
On the other hand, in order to measure the density of an examination image accurately, it is needed that the image forming area of an examination image is in accord with the detecting area of a sensor. Normally, a size of an examination image is set larger than an actually required size by taking a solid difference of the main body of a copying machine and a solid difference of a sensor into consideration so that the examination image forming area certainly agrees with the detecting area of a sensor.
The solid difference of the main body of a copying machine referred to here denotes a fluctuation between a plurality of same model of copying machines manufactured at a plant, for instance, the different mounting position of a photo-conductive drum or mounting position of a sensor.
The solid difference of a sensor denotes a fluctuation in the width of detecting areas or a fluctuation in the sensitivity of sensors.
However, when detecting a change in image quality using an unnecessarily large sized examination image as described above, the consumption of developer will increase unnecessarily and becomes a problem from the economical viewpoint.
Further, for the purpose of reducing consumption of developer by reducing an examination image size, it is possible to manually adjust the examination image forming position and the sensor detecting position for each copying machine. In this case, however, much adjusting time is needed and the drop of manufacturing efficiency and increase in manufacturing cost will result.